Fluoropolymers have been long known and have been used in a variety of applications because of several desirable properties such as heat resistance, chemical resistance, weatherability, UV-stability etc. . . . The various applications of fluoropolymers are for example described in “Modem Fluoropolymers”, edited by John Scheirs, Wiley Science 1997. Fluoropolymers include homo and co-polymers of a gaseous fluorinated olefin such as tetrafluoroethylene (TFE), chlorotrifluoroethylene (CTFE) and/or vinylidene fluoride (VDF) with one or more comonomers such as for example hexafluoropropylene (HFP) or perfluorovinyl ethers (PVE) or non-fluorinated olefins such as ethylene (E) and propylene (P). The term “copolymer” in connection with the present invention should generally be understood to mean a polymer comprising repeating units derived from the recited monomers without excluding the option of other further repeating units being present that derive from other monomers not explicitly recited. Accordingly, for example the term ‘copolymer of monomers A and B’ includes binary polymers of A and B as well as polymers that have further monomers other than A and B such as for example terpolymers.
Fluoropolymers include melt-processible and non-melt-processible polymers. For example, polytetrafluoroethylene and copolymers of tetrafluoroethylene with small amounts (e.g. not more than 0.5% by weight) of a comonomer are generally not melt-processible with conventional equipment used for the processing of thermoplastic polymers because of their high molecular weight and their high melt viscosity. Accordingly, for these non-melt-processible fluoropolymers, special processing techniques have been developed to allow forming these fluoropolymers into desired articles and shapes.
Melt-processible thermoplastic fluoropolymers are also known and these can be obtained from various combinations of fluorinated and/or non-fluorinated monomers. As they are melt-processible, they can be processed with equipment typically used for the processing, of thermoplastic polymers, such as e.g. molding or extrusion. Melt-processible thermoplastic fluoropolymers include generally amorphous fluoropolymers and fluoropolymers that have substantial crystallinity. Fluoropolymers that are generally amorphous are typically used to make fluoroelastomers by curing or vulcanizing the fluoropolymer. Although, the elastomeric properties generally are obtained after curing, the fluoropolymers used for making the fluoroelastomer are often also called fluoroelastomer. Melt-processible thermoplastic fluoropolymers that have substantial crystallinity and that accordingly have a clearly detectable and prominent melting point are known in the art as fluorothermoplasts. They typically have a melting point between 100° C. and 320° C. depending on their monomer composition.
Examples of fluorothermoplasts include copolymers of TFE and E (ETFE), copolymers of TFE and HFP (FEP), copolymers of TFE, HFP and VDF (THV) and perfluoroalkoxy copolymers (PFA). Examples of applications of fluorothermoplasts include for example coating applications such as for example for coating outdoor fabric and use as insulating material in wire and cable insulation. Further applications of fluorothermoplasts include making of tubes such as for example fuel hoses, extrusion of films and injection molded articles.
The rate of extrusion of fluorothermoplast is limited to the speed at which the polymer melt undergoes melt fracture. If the rate of extrusion exceeds the rate at which melt fracture occurs (known as critical shear rate), an undesired rough surface of the extruded article is obtained. Using an extrusion die with a relatively large orifice and then drawing the extruded melt to the desired final diameter may increase the process rate of fluorothermoplasts. Herein, the melt draw is commonly characterized by the draw down ratio calculated as the ratio of the cross-sectional area of the die opening to the ratio of the cross-sectional area of the finished extrudate. To obtain a high draw down ratio e.g. in the order of 85 to 100, the polymer melt should exhibit a sufficiently high elongational viscosity. Otherwise the cone stability of the polymer melt in the extrusion will be insufficient, which results in undesired diameter variations of the extruded article as well as frequent cone-breaks.
Accordingly, there exists a continuous need for fluorothermoplasts that can be melt-processed at higher shear rates and that have a high elongational viscosity. Various attempts have been made in the art to obtain such fluorothermoplasts or compositions thereof that can be faster processed.
A known approach in the art is to substantially broaden the molecular weight distribution (MWD) thereby increasing the critical shear rate. As disclosed in DE-A-2613795, DE-A-2613642, EP-A-88414 and EP-A-362868, FEP polymers that have a broad MWD ensure relatively fast processing at relatively high shear rates. WO 00/69969 teaches that the critical shear rate of a THV terpolymer can be efficiently increased if the fluoropolymer composition contains a small fraction of ultra high molecular weights (besides a larger fraction of low molecular weights). The MWD of such a fluoropolymer composition appears considerably asymmetrical. Unfortunately, the gain in critical shear rate is usually to the expense of weaker overall mechanical properties such flex life endurance.
In DE-A-2710501, EP-A-75312, WO 02/00741 and EP 0845147, the modification with a particular comonomer, such as perfluoro vinylethers (PVE) is taught to yield retention of necessary mechanical properties while increasing the processing speed of fluorothermoplasts. But, the additional incorporation of PVEs into fluorothermoplasts increases the manufacturing costs, which may not be desired. Furthermore the formation of die deposits (“die drool”) may occur, particularly with a broad MWD of the fluorothermoplast. In fast extrusion procedures, such as wire & cable insulation, large accumulation of die deposits separate from the die and may cause break-off of the melt cone (“cone-break”) and thus interruption of the production, and also interruption of the continuous cable.
It can thus be seen from the above that the solutions taught in the prior art have caused other disadvantages such as weaker mechanical properties, higher manufacturing costs and/or other process limitations. Further, the elongational viscosity characteristics (which are of primary importance for processes involving high draw-down ratios) are only little improved by a broad MWD taught for improving the critical shear rate.
In EP 208305 it is taught that the processing of thermoplastic, non-thermosettable copolymers of tetrafluoroethylene can be improved by copolymerizing a small amount of a iodo(perfluoroalkyl)ethylene. In particular, it is taught that the use of 4-iodo-3,3,4,4-tetrafluorobutene-1 (ITFB) increases the critical shear rate by a factor of 2 to 3 and also improves the melt tension of the fluoropolymer. It is speculated in this publication that the effect is due to long chain branching being introduced into the polymer through the ITFB. Thus, the formed fluoropolymer would be non-linear as opposed to same fluoropolymers made without ITFB which are linear. Unfortunately, the making of ITFB involves the use of highly toxic intermediates.
Accordingly, the need still exists to find further fluorothermoplasts that have high critical shear rates and/or high draw-down ratios preferably without causing other disadvantages such as reduced mechanical properties, increased cost and/or causing other processing disadvantages. It would further be desired that the making of such fluorothermoplasts does not involve the use of highly toxic compounds or compounds the manufacturing of which involves toxic components. Desirably, the thermal stability of the fluorothermoplasts is unaffected or improved and the fluorothermoplast can be readily manufactured in an environmentally friendly way preferably through aqueous emulsion polymerization.